Exhaust gas emitted from an internal combustion system such as an automobile engine comprises toxic components such as particulate matter (PM), hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Conventionally, a wall-flow type exhaust gas purification catalyst (wall-flow exhaust gas purification catalyst) is used to efficiently eliminate these exhaust components.
A wall-flow exhaust gas purification catalyst comprises entrance cells with open ends on the exhaust inlet side, exit cells with open ends on the exhaust outlet side, and porous partition walls (ribbed walls) to separate the two types of cells. The exhaust gas purification catalyst comprises a catalytic layer comprising a catalytic metal and a support that carries the catalytic metal. Exhaust gas emitted from the internal combustion system flows via the exhaust inlet-side ends into the entrance cells, passes through micro pores of the porous partition walls, and flows out of the exhaust outlet-side ends of the exit cells. When the exhaust is in contact with the catalytic layer (catalytic metal), the exhaust components are purified (detoxified).
Conventional art literature related to this includes Patent Documents 1 and 2. For instance, Patent Document 1 discloses an exhaust gas purification catalyst comprising catalytic layers in a two-layer configuration. In particular, it discloses an exhaust gas purification catalyst comprising a Pd-containing catalytic layer throughout the interior of its partition walls and further comprising another Rh-containing catalytic layer on the surface of its partition walls in contact with the entrance cells to completely cover the first catalytic layer.